Please don't make generalized criticisms about people who play a specific role. There are plenty of good and bad tanks, just like other roles.

A couple days ago in DF some tank raged at me because they thought a bad ring meant I wouldn't be able to help tank, even though my average ilvl for the dungeon was fine.
Yesterday I met a tank who kept a positive attitude while we died countless times helping new people learn and get through Hydra.
Today I worked with a couple DF Garuda EX tanks who worked on strategy with the group and died over and over as people learned the fight. No complaints or rage quits.

In terms of opportunity cost of leaving, another thing to think about is that if you play as a tank or a healer, you (often) have significantly more control over whether the party succeeds or not. If you are a good tank, you don't have to deal with bad tanks. If you are a good healer, you don't have to deal with bad healers. If a group has a good tank and a good healer, it usually only needs acceptable dps to get through the challenge.

So, if you're a dps and you encounter a bad tank or healer, it may make sense to bail. Even though you'll have to wait longer for a new group, your current group doesn't have a good chance to succeed. If you're a good healer or tank, it's not worth it as much to drop since the party has a decent chance just because you're in it.